1. Technical Field of the Invention
This invention is concerned with a fail-safe thermal protective circuit for a heater element employed in a hyperemia-inducing physiological sensor.
2. Description of the Prior Art
For medical diagnostic purposes, various types of physiological sensors may be employed. Such sensors are used for transcutaneous measurement and monitoring of various physiological phenomena, without puncturing the skin of the patient. Typical applications are measurements of blood flow, blood oxygen content, sugar content, alcohol content, carbon dioxide content, blood pH, etc. Typically, such devices are attached to the patient externally. The sensor is heated by an internal heating element for the purpose of local hyperemization at the site of the sensor.
Usually the internal heating element is electrical, although fluid-heated elements are known. Ordinarily the desired temperature range is on the order of 37 to 45 degrees C. (98.6 degrees to 113 degrees F.) and is controlled by a suitable thermostat. It is preferable that the sensor itself be constrained to minimal dimensions. Accordingly, only the sensor elements, the heater and a heat sensor such as a small thermistor are mounted in the physiological sensor. The heater controls such as a thermostat, temperature indicator and heater power supply are mounted in a separate remote control module. The control module is connected to the physiological sensor by a small, multiconductor cable.
With any electrical control device, there is always the danger of a control circuit malfunction. Thus, if the thermostat itself were to malfunction, or if the temperature sensing lines from the thermistor failed, the patient would be endangered if the physiological sensor overheated.
It is known to add a second, backup temperature sensor and thermostat to take over in the event that the primary temperature-control circuit fails. However such an arrangement requires additional conductors as well as additional complexity in the remote control module.
Accordingly, it is an object of this invention to provide a thermal protective circuit that is built into the sensor itself, that will automatically cut off the heating element in the sensor independently of the heater-control system in the event of a catastrophic failure of the conventional thermostatic temperature-control circuitry.